Densely packed incoherent wavelength division multiplex (WDM) lightwave communication systems are attractive over coherent (heterodyne) communication systems because the incoherent system uses the large wavelength (frequency) domain available in an optical fiber by assigning different wavelengths to different channels in the communication system. A key component in an incoherent WDM lightwave communication system is the lightwave receiver which provides filtering, amplification, and detection of the lightwave signals. If direct detection is used instead of coherent detection, tunable optical filters will be needed to separate the different multiplexed wavelengths (channels) both for routing and final detection purposes. A WDM receiver using direct optical detection must optically filter the multi-frequency WDM signal to pass only the desired channel or channels to the direct optical detector. A tunable optical filter therefore has the function of selecting a predetermined wavelength light signal from the plurality of multiplexed lightwave signals. Thus, the goal of a tunable optical filter is to select one channel (or several channels) in a given incoming wavelength multiplexed optical signal and block the other channels from passing through the filter.
Several different optical filters have been developed. Fabry-Perot filters with mechanical tuning, i.e., a piezo-electric element, have been developed. However, the use of mechanically operated filters has several disadvantages. First, optical filters with moving components are typically bulky and are expensive to produce. In addition, optical filters with moving components have poor reliability and lower switching speeds than electronically controlled optical filters.
Optical filters based on acousto-optic TE/TM mode conversion and waveguides have also been developed. However, these filters require a relatively complex drive circuit to generate the required acoustic waves. In addition, the filters are relatively large with a length on the order of one to two centimeters. Optical filters based on semiconductor distributed feedback (DFB) laser diodes and multi-section Fabry-Perot laser diodes have also been developed. Such distributed feedback semiconductor laser structures are operated with a bias current set below a threshold current for lasing. While these devices have the advantage of having gain, they also have several disadvantages. First, the tuning range of these distributed feedback semiconductor laser devices is small at less than two nanometers at a wavelength of 1.5 micrometers for a InGaAsP/InP device. Furthermore, they have a very narrow bandwidth because of the gain and they are susceptible to saturation effects if the incoming signal power is too high.
Optical filters based upon a distributed-Bragg reflection (DBR) laser structure have also been developed. The distributed-Bragg reflection laser structure is employed as an integrated receiver within a lightwave communication system. The DBR laser structure is biased electrically below the lasing threshold to operate as a multifunctional element by performing the integrated operation of resonant amplification and filtering of the received lightwave signals. By electrically biasing the Bragg section of the DBR laser structure, it is possible to tune the filter so as to select the desired wavelength. The DBR laser structure has a larger tuning range, up to 15 nanometers, than the DFB laser diodes, and it can be made insensitive to the power of the input signal by removing the gain section from the filter. However, the DBR laser structure operates using reflection wherein the output signal is reflected back in the input waveguide, through the input signal, which results in loss penalties when the signals are separated.